Adenosine is a nucleoside that influences the activity of numerous tissues by binding to specific cell surface receptors. Growing evidence suggests that A1 adenosine receptors (A1ARs) are important neuromodulators that are widely distributed in the mammalian brain. A1ARs are believed to tonically inhibit neural activity, and activation of A1ARs can stop seizures. A1ARs may modulate the action of excitatory neurotransmitters, and A1AR activation also may help protect the brain against injury. Increasing evidence suggests that the hippocampal formation is an important site of A1AR action. In comparison with what is known about other receptor systems in the brain, surprisingly little is known about the neural adenosinergic system. Using contemporary molecular and biochemical techniques, we propose to examine the sites and mechanisms of A1AR action in the hippocampus. First, we will identify the cellular sites of A1ARs in human hippocampus. We will use immunohistochemistry, receptor autoradiography and in situ hybridization for these studies. Second, we will examine the structural basis of how the human A1AR interacts with agonists and antagonist. Using chimeric receptors, we will identify transmembrane domains of that are important for ligand binding. We will then use small chimeric constructs and site-directed mutagenesis to localize specific amino acids that are important for binding agonists, antagonists, and adenosine enhancers. Third, we will examine if A1AR activation alone influences hippocampal physiology, and examine if adenosine itself acts predominantly via A1ARs. Using rat hippocampal slices, we will examine if A1ARs inhibit excitatory amino acid release and examine if A1ARs influence hippocampal gene expression. A1ARs may play a very important role in the pathogenesis and treatment of nervous system disorders including epilepsy. Understanding the basic mechanisms of human A1AR action in hippocampus, will allow us to better understand the role of this important receptor system in human neural physiology.